doc: Fix dead link in gitian-building
doc: Fix dead link in gitian-building

- Link to Debian 8.1 netinstall ISO is dead, changed to valid 8.2
- Changed checksum to 8.2 netinstall ISO checksum (http://cdimage.debian.org/debian-cd/8.2.0/amd64/iso-cd/SHA256SUMS)
- Verified CD checksum, checksum file's signing key

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